1. Field of the Invention
This invention is concerned with a process for converting synthesis gas comprising hydrogen and carbon oxides to form a product comprising a substantial proportion of organic oxygenates. In one aspect, this invention is concerned with a process for converting synthesis gas comprising hydrogen and carbon oxides to form mixtures of oxygenates and hydrocarbons, which are limited compositionally in being virtually free of C.sub.11.spsb.+ compounds. In another aspect, this invention is concerned with providing a novel catalyst composition for the conversion of synthesis gas to a product comprising a substantial proportion of organic oxygenates, which is limited compositionally in being virtually free of C.sub.11.spsb.+ compounds.
2. Prior Art
Processes for the conversion of coal and other hydrocarbons such as natural gas to a gaseous mixture consisting essentially of hydrogen and carbon monoxide, or of hydrogen and carbon dioxide, or of hydrogen and carbon monoxide and carbon dioxide, are well known. Although various processes may be employed for the gasification of carbonaceous fuels, those of major importance depend either on the partial combustion of the fuel with an oxygen-containing gas or on the high temperature reaction of the fuel with steam, or on a combination of these two reactions. An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels, is given in the ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433 (1966), Interscience Publishers, New York, N.Y., the contents of which are herein incorporated by reference. The techniques for gasification of coal or other solid, liquid or gaseous fuel are not considered to be per se inventive here.
It would be very desirable to be able to effectively convert synthesis gas, and thereby coal and natural gas, to highly valued fuels such as motor gasoline with high octane number and chemical intermediates. It is well known that synthesis gas will undergo conversion to form reduction products of carbon monoxide, such as hydrocarbons and alcohols, at from about 300.degree. F. to about 850.degree. F. under pressure from about 1 to 1000 atmospheres, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been most extensively studied, produces a range of liquid hydrocarbons, a portion of which have been used as low octane gasoline. The types of catalysts that have been studied for this and related processes include those based on metals or oxides of zinc, iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium.
Catalysts based on ZnO are particularly suited for the production of methanol and dimethyl ether. Catalysts based on Fe, Co, and Ni, and especially Fe, are particularly suited for the production of oxygenated and hydrocarbon products that have at least one carbon-to-carbon bond in their structure. With the exception of ruthenium, all practical synthesis catalysts contain chemical and structural promoters. These promoters include copper, chromia, alumina and alkali. Alkali is of particular importance with iron catalysts, since it greatly enhances the conversion efficiency of the iron catalyst. Supports such as kieselguhr sometimes act beneficially.
The catalyzed reduction of carbon monoxide or carbon dioxide by hydrogen produces various oxygenated and hydrocarbon products, depending on the particular catalyst and reaction conditions chosen. The products that are formed include methanol; dimethyl ether; acetone; acetic acid; normal propyl alcohol; higher alcohols; methane; gaseous, liquid and solid olefins and paraffins. It should be noted that this spectrum of products consists of aliphatic compounds; aromatic hydrocarbons either are totally absent or are formed in minor quantities.
In general, when operating at the lower end of the temperature range, i.e. from about 300.degree. F. to about 500.degree. F., in the reduction of carbon monoxide, and with pressures greater than about 20 atmospheres, thermodynamic considerations suggest that aliphatic hydrocarbons are likely to form in preference to their aromatic counterparts. Furthermore, in some catalytic systems it has been noted that aromatic hydrocarbon impurities in the synthesis gas inactivate the synthesis catalyst, and one may speculate that a number of known synthesis catalysts intrinsically are not capable of producing aromatic hydrocarbons.
The wide range of catalysts and catalyst modifications disclosed in the art and an equally wide range of conversion conditions for the reduction of carbon monoxide by hydrogen provide considerable flexibility toward obtaining selected boiling range products. Nonetheless, in spite of this flexibility, it has not proved possible to make such selections so as to produce oxygenates or mixtures of oxygenates and hydrocarbons which are limited compositionally in being virtually free of C.sub.11.spsb.+ compounds. A review of the status of this art is given in Carbon Monoxide-Hydrogen Reactions, ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Edited by Kirk-Othmer, Second Edition, Volume 4, pages 446-448, Interscience Publishers, New York, N.Y., the text of which is incorporated herein by reference.
Recently it has been discovered that synthesis gas may be converted to oxygenated organic compounds and these then converted to higher hydrocarbons, particularly high octane gasoline, by catalytic contact of the synthesis gas with a carbon monoxide reduction catalyst followed by contacting the conversion products so produced with a special type of zeolite catalyst in a separate reaction zone. This two-stage conversion is described in U.S. Pat. No. 4,076,761.
Another process to produce high octane gasoline is disclosed in U.S. Pat. No. 3,972,958. In this process, coal is gasified and the resultant synthesis gas from the gasification is converted into high octane aromatic gasoline and light hydrocarbon gases.
The conversion of synthesis gas to hydrocarbon mixtures is described in U.S. Pat. Nos. 4,086,262 and 4,096,163. These patents involve the use of acidic crystalline zeolites in admixture with carbon oxide reducing components, such as Fischer-Tropsch catalysts. Conversion of synthesis gas to hydrocarbon mixtures is also described in U.S. Pat. No. 4,157,338.
Copending U.S. patent application Ser. No. 926,987 filed July 21, 1978, now U.S. Pat. No. 4,172,843, describes conversion of syngas to olefinic naphtha utilizing a catalyst comprising an iron containing Fischer-Tropsch Component and a substantially non-acidic ZSM-5 type zeolite.
Compositions of iron, cobalt or nickel deposited in the inner absorption regions of crystalline zeolites are described in U.S. Pat. No. 3,013,990. Attempts to convert synthesis gas over X-zeolite base exchanged with iron, cobalt and nickel are described in Erdoel und Kohle--ERDGAS, PETROCHEMIE: BRENNSTOFF-CHEMIE, Volume 25, No. 4, pages 187-188, April 1972.
It is an object of the present invention to provide an improved process for converting fossil fuels to mixtures of hydrocarbons and oxygenates containing large quantities of high desirable constituents. It is a further object of this invention to provide a more efficient method for converting a mixture of gaseous carbon oxides and hydrogen to form a product comprising a substantial proportion of organic oxygenates. It is a further object of this invention to provide an improved method for converting synthesis gas to mixtures of hydrocarbons and oxygenates which are virtually free of C.sub.11.spsb.+ compounds. It is a further object of this invention for converting synthesis gas to high octane gasoline and oxygenates which may be valuable as chemical intermediates. It is a further object of this invention to provide novel catalysts for the conversion of synthesis gas.